U.S. patent number 7,569,510 [Application Number 11/698,192] was granted by the patent office on 2009-08-04 for catalysts to reduce carbon monoxide such as in the mainstream smoke of a cigarette.
This patent grant is currently assigned to Philip Morris USA Inc.. Invention is credited to Sarojini Deevi, Padmanabha Reddy Ettireddy.
United States Patent |
7,569,510 |
Deevi , et al. |
August 4, 2009 |
Catalysts to reduce carbon monoxide such as in the mainstream smoke
of a cigarette
Abstract
Catalysts for the conversion, or oxidation, of carbon monoxide
to carbon dioxide. Cigarettes with filters containing the catalysts
have acceptable resistance to draw. Additionally, the catalysts can
be used to reduce the concentration of carbon monoxide from a
vehicle exhaust emission, a gas used in a CO.sub.2 laser, a gas
used in a fuel cell and/or ambient air undergoing air
filtration.
Inventors: |
Deevi; Sarojini (Midlothian,
VA), Ettireddy; Padmanabha Reddy (Richmond, VA) |
Assignee: |
Philip Morris USA Inc.
(Richmond, VA)
|
Family
ID: |
38437753 |
Appl.
No.: |
11/698,192 |
Filed: |
January 26, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070204870 A1 |
Sep 6, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60776679 |
Feb 27, 2006 |
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Current U.S.
Class: |
502/240; 502/355;
502/350; 502/325; 502/304; 502/180; 131/360; 131/331 |
Current CPC
Class: |
B01J
37/031 (20130101); B01J 23/52 (20130101); A24D
3/16 (20130101); B01D 53/864 (20130101); A24B
15/287 (20130101); B01J 35/0013 (20130101); A24B
15/288 (20130101); B01J 37/16 (20130101); A24B
15/28 (20130101); A24B 15/286 (20130101); Y02P
70/50 (20151101); B01D 2255/2092 (20130101); B01D
2255/20715 (20130101); B01D 2255/206 (20130101); B01D
2255/106 (20130101); B01D 2255/9202 (20130101); B01D
2255/1021 (20130101); B01D 53/9477 (20130101); B01D
2255/2047 (20130101); Y02P 70/66 (20151101); B01D
2255/20707 (20130101); B01D 2251/11 (20130101); B01D
2255/1023 (20130101); B01D 2257/502 (20130101); B01D
2255/20738 (20130101) |
Current International
Class: |
B01J
21/12 (20060101) |
Field of
Search: |
;502/180,240,325,350,355,304 ;131/331,360 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0306945 |
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Mar 1989 |
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EP |
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1116585 |
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Jun 1986 |
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GB |
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WO 95/51401 |
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Nov 1998 |
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WO |
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WO 2005/118133 |
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Dec 2005 |
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WO |
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Other References
Rand et al., "Carbon-Ceramic Alloys", Design and Control of
Structure of Advanced Carbon Materials for Enhanced Performance,
319-337 (2001) Kluwer Academic Publishers, Netherlands. cited by
other .
International Search Report and Written Opinion for International
Patent Application No. PCT/IB2007/001775. cited by other .
International Preliminary Report on Patentability dated Sep. 2,
2008 for PCT/IB2007/001775. cited by other.
|
Primary Examiner: Wood; Elizabeth D
Attorney, Agent or Firm: Buchanan Ingersoll & Rooney
PC
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. 119 to U.S.
Provisional Patent Application No. 60/776,679 filed on Feb. 27,
2006, the entire content of which is hereby incorporated by
reference.
Claims
What is claimed is:
1. A catalyst comprising noble metal nanoparticles on non-noble
metal oxides incorporated in a porous support of mesoporous silica
wherein the catalyst is catalytically active for oxidation of
carbon monoxide and wherein the noble metal nanoparticles are
selected from the group consisting of gold, palladium, platinum and
mixtures thereof and the non-noble metal oxides are selected from
the group consisting of titania, alumina, ceria, zirconia, iron
oxide, zinc oxide, magnesium oxide, and mixtures thereof.
2. The catalyst of claim 1, wherein the mesoporous silica is
selected from the group consisting of SBA-15, SBA-16, MCM-41, and
MCM-48.
3. A catalyst comprising noble metal nanoparticles on non-noble
metal oxides incorporated in a porous support wherein the catalyst
is catalytically active for oxidation of carbon monoxide and
wherein the noble metal nanoparticles are gold, the non-noble metal
oxides are selected from the group consisting of ceria, zirconia,
iron oxide, magnesium oxide, and mixtures thereof and the porous
support is mesoporous silica.
4. The catalyst of claim 1, comprising 1-2 weight %
gold-titania.
5. A cigarette comprising a filter containing the catalyst of claim
1.
6. A method of making a catalyst catalytically active for oxidation
of carbon monoxide comprising: treating a catalyst comprising noble
metal nanoparticles on non-noble metal oxides incorporated in a
support to develop porosity in the support, wherein the noble metal
nanoparticles are gold, the non-noble metal oxides are selected
from the group consisting of ceria, zirconia, iron oxide, magnesium
oxides, and mixtures thereof and the porous support is mesoporous
silica.
7. The method of claim 6, wherein the catalyst is subjected to an
oxidation treatment to develop porosity in the support by heating
the catalyst in the presence of an oxidizing agent selected from
the group consisting of steam, carbon dioxide, air and mixtures
thereof.
8. A cigarette comprising a filter containing a catalyst, the
catalyst comprising noble metal nanoparticles on non-noble metal
oxides incorporated in a porous support, wherein the catalyst is
catalytically active for oxidation of carbon monoxide, the noble
metal nanoparticles are selected from the group consisting of gold,
palladium, platinum and mixtures thereof, the non-noble metal
oxides are selected from the group consisting of titania, alumina,
ceria, zirconia, iron oxide, zinc oxide, magnesium oxide, and
mixtures thereof, the support is selected from the group consisting
of carbon beads, activated carbon, mesoporous silica, mesoporous
titania, mesoporous alumina, and mesoporous ceria and the
mesoporous silica is selected from the group consisting of SBA-15,
SBA-16, MCM-41, and MCM-48.
Description
SUMMARY
In one embodiment, a catalyst catalytically active for oxidation of
carbon monoxide comprises noble metal nanoparticles on non-noble
metal oxides incorporated in a porous support.
In another embodiment, a method of making a catalyst catalytically
active for oxidation of carbon monoxide comprises treating a
catalyst comprising noble metal nanoparticles on non-noble metal
oxides incorporated in a support to develop porosity in the
support.
In yet another embodiment, a catalyst catalytically active for
oxidation of carbon monoxide comprises noble metal nanoparticles on
non-noble metal oxides incorporated in a silica gel support.
In a further embodiment, a method of making a catalyst
catalytically active for oxidation of carbon monoxide comprises
incorporating noble metal nanoparticles on non-noble metal oxides
in a silica gel support.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the structure of a 34 mm long cigarette filter
having adjacent sections of an 8 mm cellulose acetate (CA) filter
segment, an 8 mm CA segment, a 12 mm cavity containing catalyst,
and a 6 mm CA segment, the filter attached via tipping paper to a
49 mm long tobacco rod.
FIG. 2 illustrates the structure of a 34 mm long dual cavity
cigarette filter having adjacent sections of a 7 mm CA filter
segment, a 5 mm CA segment, a 6 mm cavity containing catalyst, a 5
mm CA segment, a 6 mm cavity containing catalyst, and a 5 mm CA
segment, the filter attached via tipping paper to a 49 mm long
tobacco rod.
FIG. 3 shows the results of carbon monoxide oxidation at room
temperature using 100 mg of 2 weight % Au--TiO.sub.2 Hombikat
catalyst.
DETAILED DESCRIPTION
Smoking articles, such as cigarettes or cigars, produce both
mainstream smoke during a puff and sidestream smoke during static
burning. One constituent of both mainstream smoke and sidestream
smoke is carbon monoxide (CO). The reduction of carbon monoxide in
smoke is desirable.
Catalysts for the conversion, or oxidation, of carbon monoxide to
carbon dioxide are provided. The catalyst is preferably capable of
room temperature oxidation of carbon monoxide. Accordingly, the
catalyst can be incorporated into cigarette filter material. The
catalyst can also be used to reduce the concentration of carbon
monoxide from a vehicle exhaust emission, a gas used in a CO.sub.2
laser, a gas used in a fuel cell and/or ambient air undergoing air
filtration. The catalyst may be incorporated into a vehicle exhaust
emissions system in an amount effective to oxidize carbon monoxide
to carbon dioxide. The catalyst may also be used for emissions
reduction in the cold starting of an automobile engine in an amount
effective to oxidize carbon monoxide to carbon dioxide. The
catalyst may be incorporated into a CO.sub.2 laser in an amount
effective to oxidize carbon monoxide to carbon dioxide. The
catalyst can be incorporated into a fuel cell in an amount
effective to oxidize carbon monoxide to carbon dioxide. The
catalyst can be used in an air filter for the conversion of carbon
monoxide and/or indoor volatile organic compounds.
"Smoking" of a cigarette means the heating or combustion of the
cigarette to form smoke, which can be drawn through the cigarette.
Generally, smoking of a cigarette involves lighting one end of the
cigarette and, while the tobacco contained therein undergoes a
combustion reaction, drawing the cigarette smoke through the mouth
end of the cigarette. The cigarette may also be smoked by other
means. For example, the cigarette may be smoked by heating the
cigarette and/or heating using electrical heater means, as
described in commonly-assigned U.S. Pat. Nos. 6,053,176; 5,934,289;
5,591,368 and 5,322,075.
The term "mainstream" smoke refers to the mixture of gases passing
down the tobacco rod and issuing through the filter end, i.e., the
amount of smoke issuing or drawn from the mouth end of a cigarette
during smoking of the cigarette.
In addition to the constituents in the tobacco, the temperature and
the oxygen concentration are factors affecting the formation and
reaction of carbon monoxide and carbon dioxide. The total amount of
carbon monoxide formed during smoking comes from a combination of
three main sources: thermal decomposition (about 30%), combustion
(about 36%) and reduction of carbon dioxide with carbonized tobacco
(at least 23%). Formation of carbon monoxide from thermal
decomposition, which is largely controlled by chemical kinetics,
starts at a temperature of about 180.degree. C. and finishes at
about 1050.degree. C. Formation of carbon monoxide and carbon
dioxide during combustion is controlled largely by the diffusion of
oxygen to the surface (k.sub.a) and via a surface reaction
(k.sub.b). At 250.degree. C., k.sub.a and k.sub.b, are about the
same. At about 400.degree. C., the reaction becomes diffusion
controlled. Finally, the reduction of carbon dioxide with
carbonized tobacco or charcoal occurs at temperatures around
390.degree. C. and above.
During smoking there are three distinct regions in a cigarette: the
combustion zone, the pyrolysis/distillation zone, and the
condensation/filtration zone. While not wishing to be bound by
theory, it is believed that catalysts can target the various
reactions that occur in different regions of the cigarette during
smoking.
First, the combustion zone is the burning zone of the cigarette
produced during smoking of the cigarette, usually at the lighted
end of the cigarette. The temperature in the combustion zone is in
the range from about 700.degree. C. to about 950.degree. C., and
the heating rate can be as high as 500.degree. C./second. Because
oxygen is being consumed in the combustion of tobacco to produce
carbon monoxide, carbon dioxide, water vapor, and various organics,
the concentration of oxygen is low in the combustion zone. The low
oxygen concentrations coupled with the high temperature leads to
the reduction of carbon dioxide to carbon monoxide by the
carbonized tobacco. The combustion zone is highly exothermic and
the heat generated is carried to the pyrolysis/distillation
zone.
The pyrolysis zone is the region behind the combustion zone, where
the temperatures range from about 200.degree. C. to about
600.degree. C. The pyrolysis zone is where most of the carbon
monoxide is produced. The major reaction is the pyrolysis (i.e.,
thermal degradation) of the tobacco that produces carbon monoxide,
carbon dioxide, smoke components and/or carbon using the heat
generated in the combustion zone.
In the condensation/filtration zone the temperature ranges from
ambient to about 60.degree. C. The major process in this zone is
the condensation/filtration of the smoke components. Some amount of
carbon monoxide and carbon dioxide diffuse out of the cigarette and
some oxygen diffuses into the cigarette. The partial pressure of
oxygen in the condensation/filtration zone does not generally
recover to the atmospheric level. In the condensation/filtration
zone, the catalyst can promote oxidation of carbon monoxide to
carbon dioxide in the presence of oxygen, at temperatures from
ambient to about 60.degree. C.
Incorporating catalysts in a preferred cigarette filter discussed
herein results in minimal changes in smoke chemistry, minimizes
additional products due to combustion, and requires only small
amounts of catalyst. Catalysts tested for potential inclusion in
cigarette filters include gold/titania deposited on activated
carbon (PICA Carbon, Levallois, France; mesh size of 20.times.50
mesh or 40.times.60 mesh), carbon beads (100-700 microns in
diameter), and silica gel beads.
The cigarette of FIG. 1 contains a 49 mm long tobacco rod and a 34
mm long filter having adjacent sections of an 8 mm CA filter
segment, an 8 mm CA segment, a 12 mm cavity containing catalyst,
and a 6 mm CA segment. The cigarette of FIG. 2. contains a 49 mm
long tobacco rod and a 34 mm long dual cavity filter having
adjacent sections of a 7 mm CA filter segment, a 5 mm CA segment, a
6 mm cavity containing catalyst, a 5 mm CA segment, a 6 mm cavity
containing catalyst, and a 5 mm CA segment. It is contemplated that
the catalysts disclosed herein may be contained in only one of the
dual cavities or both of the dual cavities. In FIG. 1 and FIG. 2,
the filter is attached to the tobacco rod via tipping paper. Flavor
may optionally be incorporated into the filter, preferably
downstream of the catalyst.
The catalysts disclosed herein, incorporated in cigarette filters,
can be effective for the removal of carbon monoxide, nitric oxide,
and/or provide reduction in total particulate matter (TPM).
However, the size of the catalyst particles should be controlled
such that fine catalyst particles, which can result in high
resistance to draw (RTD), possibly as a result of clogging of
support pores, are not incorporated. In order to further avoid high
RTD, the catalysts are preferably anchored in porous supports,
which comprise interconnected pores that extend from one surface of
a support to another. The catalyst is preferably anchored on
external surfaces of the support and on the periphery of the pores
in order to maximize contact between the catalyst and carbon
monoxide.
Thus, the catalyst for the conversion of carbon monoxide to carbon
dioxide may comprise noble metal nanoparticles on non-noble metal
oxides incorporated in a porous support. Accordingly, as used
herein, the term "catalyst" refers to noble metal nanoparticles on
non-noble metal oxides. The catalyst is incorporated in a porous
support. Preferably, the support is macroporous, which refers to a
material with a pore size of about 500 .ANG. or larger. The support
may comprise, for example, carbon beads or activated carbon,
preferably macroporous carbon beads or macroporous activated
carbon. The carbon beads may comprise phenolic resin, for example
carbonized phenolic resin. Alternatively, the support may comprise
mesoporous silica, mesoporous titania, mesoporous alumina, or
mesoporous ceria. Mesoporous refers to a material with a pore size
of about 20-500.ANG.. The mesoporous silica may comprise, for
example, SBA-15, SBA-16, MCM-41, or MCM-48. Examples of noble metal
nanoparticles suitable for use in the catalyst include, for
example, gold, palladium, platinum and mixtures thereof. Examples
of non-noble metal oxides suitable for use in the catalyst include,
for example, titania, alumina, ceria, zirconia, iron oxide, zinc
oxide, magnesium oxide, and mixtures thereof. The noble metal
nanoparticles on non-noble metal oxides may comprise, for example,
1-5 weight % gold-ceria, 2-5 weight % palladium-ceria,
silver/silver oxide-ceria, copper oxide/zinc oxide-ceria, 20 weight
% copper oxide-80 weight % Hombikat titania, 10 weight % copper
oxide-10 weight % manganese oxide-80 weight % Hombikat titania.
Preferably, the noble metal nanoparticles on non-noble metal oxides
comprises 0.5-5 weight % gold-titania, more preferably 1-2 weight %
gold-titania.
"Nanoparticles" refers to a class of materials whose distinguishing
feature is that their average diameter, particle or other
structural domain size is below about 100 nanometers. The
nanoparticles can have an average particle size less than about 100
nm, preferably less than about 50 nm, more preferably less than
about 15 nm, even more preferably less than about 10 nm, and most
preferably less than about 7 nm. Nanoparticles have very high
surface area to volume ratios, which makes them attractive for
catalytic applications.
A catalyst comprising noble metal nanoparticles on non-noble metal
oxides incorporated in a support may be treated to develop porosity
in the support. The treatment to develop porosity preferably
comprises an oxidation reaction at high temperatures, with an
oxidizing agent such as, for example, steam, carbon dioxide, air
and mixtures thereof. By treating the catalyst to develop porosity
in the support, as opposed to depositing noble metal nanoparticles
on non-noble metal oxides on a support, the pores of the support
are not clogged. The catalyst is treated to develop porosity in the
support, such that the support is preferably macroporous. The
support may comprise, for example, carbon beads or activated
carbon. The carbon beads may comprise phenolic resin, for example,
carbonized phenolic resin. Alternatively, the support may comprise,
for example, silica, titania, alumina, or ceria, which is treated
to develop mesoporous silica, mesoporous titania, mesoporous
alumina, or mesoporous ceria, respectively. Examples of noble metal
nanoparticles suitable for use in the catalyst include, for
example, gold, palladium, platinum and mixtures thereof. Examples
of non-noble metal oxides suitable for use in the catalyst include,
for example, titania, alumina, ceria, zirconia, iron oxide, zinc
oxide, magnesium oxide, and mixtures thereof. The noble metal
nanoparticles on non-noble metal oxides may comprise, for example,
1-5 weight % gold-ceria, 2-5 weight % palladium-ceria,
silver/silver oxide-ceria, copper oxide/zinc oxide-ceria, 20 weight
% copper oxide-80 weight % Hombikat titania, 10 weight % copper
oxide-10 weight % manganese oxide-80 weight % Hombikat titania.
Preferably, the noble metal nanoparticles on non-noble metal oxides
comprises 0.5-5 weight % gold-titania, more preferably 1-2 weight %
gold-titania.
The amount of catalyst included in the cigarette filter can be
varied. For example, up to about 300 mg of catalyst can typically
be used in a cigarette or other smoking article. For example,
within the usual range, amounts such as about 20, 30, 50, 75, 100,
150, 200, or 250 mg of catalyst can be used in a cigarette. The
amount of catalyst used in a cigarette depends on the amount of
constituents in the tobacco smoke, and the amount of the
constituents that is desired to be removed from the tobacco
smoke.
The effectiveness of the catalyst incorporated into a cigarette
filter component may be impacted by catalyst diameter, with smaller
catalyst likely being more efficient. However, as noted above, the
size of the catalyst particles should be controlled such that fine
catalyst particles, which can result in high RTD, are not
incorporated. Thus, the catalyst particle size is preferably
approximately 50 to 800 microns, more preferably approximately 300
to 500 microns, preferably at an activation level equivalent to a
BET surface area in the range of 1,000 to 1,600 m.sup.2/g, more
preferably in the range of 1,100 to 1,300 m.sup.2/g.
Metal nanoparticles on metal oxides, preferably noble metal
nanoparticles on non-noble metal oxides, can be formed by such
methods as, for example, deposition-precipitation or
co-precipitation or in situ upon heating a mixture of suitable
metal precursor compounds. For example, a metal precursor, such as
gold hydroxide, can be dissolved in a suitable solvent, such as
alcohol, and mixed with a second metal precursor, such as titanium
pentane dionate. The metal precursor mixture can be heated to a
relatively low temperature, for example about 200-400.degree. C.,
wherein thermal decomposition of the metal precursors results in
the formation of metal nanoparticles on metal oxide particles that
can range in size from about 100 nm to about 500 nm.
Alternatively, nanoparticles can be formed in situ upon heating a
mixture of a suitable metal precursor compound and support. By way
of example, a metal precursor compound, such as gold hydroxide, can
be dissolved in a suitable solvent, such as alcohol, and mixed with
a dispersion of a support material, such as colloidal silica, which
can be gelled in the presence of an acid or base and allowed to dry
such as by drying in air. Acids and bases that can be used to gel
the colloidal mixture include hydrochloric acid, acetic acid,
formic acid, ammonium hydroxide and the like. When an acid
containing chlorine is used to gel the colloidal mixture,
preferably the gel is washed in de-ionized water in order to reduce
the concentration of chloride ions in the gel. The colloidal
support material can be any suitable concentration such as, for
example, 10-60 weight %, e.g., a 15 weight % dispersion or a 40
weight % dispersion. During or after gelation, the metal
precursor-colloidal silica mixture can be heated to a relatively
low temperature, for example 200-400.degree. C., wherein thermal
decomposition of the metal precursor results in the formation of
metal nanoparticles or metal oxides on silica support particles. In
place of colloidal silica, colloidal titania or a colloidal
silica-titania mixture can be used as a support. Colloidal support
particles can range in size from about 10 nm to about 500 nm.
In particular, 2 weight % Au--TiO.sub.2 Hombikat catalyst can be
prepared with Hombikat titanium dioxide, manufactured by
Sachtleben, Duisburg, Germany, by dissolving 30 weight %
tetrachloroauric acid solution in deionized water and dispersing
Hombikat in the solution. An excess of solid urea is also added to
the solution. The resulting mixture is heated slowly to
90-95.degree. C. on a hot plate with vigorous stirring. No change
in pH of the solution is observed until the temperature of the
solution reaches 90-95.degree. C., at which time the pH of the
solution increases slowly, attaining a final value of 7-8. Heating
is further continued at this pH for 1 hour in order to complete the
precipitation of metal oxides. The resulting precipitates are
filtered and washed with deionized water. The resulting solid is
then dried at room temperature. The catalyst is reduced at
150-200.degree. C. for 20 minutes in the presence of 3.6%
CO/balance Ar.
Carbon monoxide oxidation using 100 mg of the 2 weight %
Au--TiO.sub.2 Hombikat catalyst was tested with 1 L/minute of 3.6%
CO/21% O.sub.2/balance Ar at room temperature. The carbon monoxide
oxidation initiated at room temperature and 100% of the carbon
monoxide was oxidized. Results can be found in FIG. 3.
As catalysts may be deactivated by cigarette smoke as a result of
active components in cigarette smoke, further testing was done to
determine the effectiveness of the 2 weight % Au--TiO.sub.2
Hombikat catalyst in reducing CO and NO content in cigarette smoke.
First, for comparative data, percentage levels of CO, CO.sub.2, and
O.sub.2 and ppm levels of NO were measured in cigarette smoke.
Then, percentage levels of CO, CO.sub.2, and O.sub.2 and ppm levels
of NO were again measured after passing the cigarette smoke through
catalyst bed(s) containing the 2 weight % Au--TiO.sub.2 Hombikat
catalyst.
In a first trial, a catalyst bed containing 100 mg of the 2 weight
% Au--TiO.sub.2 Hombikat catalyst with a particle size of 50
microns was placed 150 cm from the cigarette. At 60.degree. C., the
catalyst bed was effective in reducing CO and NO content by 69% and
94%, respectively.
In a second trial, dual catalyst beds placed 150 cm from the
cigarette contained 200 mg (100 mg in each bed) of the 2 weight %
Au--TiO.sub.2 Hombikat catalyst with a particle size of 50 microns.
At room temperature, the catalyst beds were effective in reducing
CO and NO content by 32% and 40%, respectively.
In a third trial, a catalyst bed containing 240 mg of the 2 weight
% Au--TiO.sub.2 Hombikat catalyst with a particle size of 500-800
microns was placed 10 cm from the cigarette. At room temperature,
the catalyst bed was effective in reducing CO and NO content by 20%
and 12%, respectively.
In a fourth trial, the catalyst bed placed 10 cm from the cigarette
contained 300 mg of the 2 weight % Au--TiO.sub.2 Hombikat catalyst
with a particle size of less than 500 microns. At room temperature,
the catalyst bed was effective in reducing CO and NO content by 35%
and 29%, respectively.
In a fifth trial, 150 mg of the 2 weight % Au--TiO.sub.2 Hombikat
catalyst with a particle size of 500-800 microns was placed in the
cigarette filter. The catalyst was effective in reducing CO and NO
content by 20% and 23%, respectively.
In a sixth trial, a catalyst bed containing 300 mg of regenerated 2
weight % Au--TiO.sub.2 Hombikat catalyst with a particle size of
500-800 microns was placed 10 cm from the cigarette. At room
temperature, the catalyst bed was effective in reducing CO and NO
content by 35% and 25%, respectively. Used catalyst can be
regenerated by exposure to 3-7% CO/balance Ar at 200.degree. C. for
10 min, followed by exposure to air at room temperature for 1
hour.
Thus, using greater amounts of catalyst was found to be more
effective in reducing CO and NO content in cigarette smoke, as were
smaller catalyst particle sizes. However, in order to control the
size of the catalyst particles and avoid high RTD, the catalyst
particles are preferably pressed, sieved granules.
Silica hydrogel, also known as silica aquagel, is a silica gel
formed in water. The pores of a silica hydrogel are filled with
water. Xerogels and aerogels are examples of silica gel supports.
An xerogel is a hydrogel with the water removed. An aerogel is a
type of xerogel from which the liquid has been removed in such a
way as to minimize collapse or change in the pore structure as the
water is removed.
Use of silica gel supports, such as xerogels and/or aerogels, would
avoid catalyst powder from being loose in the cigarette, thereby
reducing potential for entrainment of catalyst powder.
Additionally, use of silica gel supports, such as xerogels and/or
aerogels, may prevent cigarette tobacco from drying out during
storage of cigarettes.
Thus, the catalyst for the conversion of carbon monoxide to carbon
dioxide may comprise noble metal nanoparticles on non-noble metal
oxides incorporated in a silica gel support. Examples of silica gel
supports suitable for use in the catalyst include, for example,
xerogels, aerogels, and mixtures thereof. Examples of noble metal
nanoparticles suitable for use in the catalyst include, for
example, gold, palladium, platinum and mixtures thereof. Examples
of non-noble metal oxides suitable for use in the catalyst include,
for example, titania, alumina, ceria, zirconia, iron oxide, zinc
oxide, magnesium oxide, and mixtures thereof. The noble metal
nanoparticles on non-noble metal oxides may comprise, for example,
1-5 weight % gold-ceria, 2-5 weight % palladium-ceria,
silver/silver oxide-ceria, copper oxide/zinc oxide-ceria, 20 weight
% copper oxide-80 weight % Hombikat titania, 10 weight % copper
oxide-10 weight % manganese oxide-80 weight % Hombikat titania.
Preferably, the noble metal nanoparticles on non-noble metal oxides
comprises 0.5-5 weight % gold-titania, more preferably 1-2 weight %
gold-titania.
Accordingly, a further embodiment of the method of making a
catalyst capable of converting carbon monoxide to carbon dioxide,
which does not exhibit high RTD when incorporated in a cigarette
filter, comprises incorporating noble metal nanoparticles deposited
on non-noble metal oxides in a silica gel support. Examples of
silica gel supports suitable for use in the method include, for
example, xerogels, aerogels, and mixtures thereof.
Silica gel can be prepared by conventional means such as by mixing
an aqueous solution of an alkali metal silicate (e.g., sodium
silicate) with a strong acid such as nitric or sulfuric acid, the
mixing being done under suitable conditions of agitation to form a
clear silica sol which sets into a hydrogel. The resulting gel can
be washed. The concentration of the SiO.sub.2 in the hydrogel is
usually in the range of about 10-60 weight %.
Washing can be accomplished simply by immersing the newly formed
hydrogel in a continuously moving stream of water which leaches out
the undesirable salts, leaving essentially pure silica (SiO.sub.2).
The pH, temperature, and duration of the wash can influence the
physical properties of the silica particles, such as surface area
and pore volume.
Metal organic decomposition (MOD) can be used to prepare
nanoparticles or metal nanoparticles on metal oxides. The MOD
process starts with a metal precursor containing the desired
metallic element dissolved in a suitable solvent. For example, the
process can involve a single metal precursor bearing one or more
metallic atoms or the process can involve multiple single metallic
precursors that are combined in solution to form a solution
mixture.
Metal nanoparticles may be incorporated into the support by various
methods, such as, for example, ion exchange, impregnation, physical
admixture, deposition-precipitation, co-precipitation, in situ
precipitation by urea hydrolysis, and/or hydrothermal methods. For
example, the metal precursor may be dissolved or suspended in a
liquid, and the support may be mixed with the liquid having the
dispersed or suspended metal precursor. The dissolved or suspended
metal precursor can be adsorbed onto a surface of the support or
absorbed into the support. The metal precursor may also be
deposited onto a surface of the support by removing the liquid,
such as by evaporation so that the metal precursor remains on the
support. The liquid may be substantially removed from the support
during or prior to thermally treating the metal precursor, such as
by heating the support at a temperature higher than the
decomposition temperature of the precursor or by reducing the
pressure of the atmosphere surrounding the support.
When the catalyst promotes oxidation of carbon monoxide, a
significant reduction in the amount of carbon monoxide can be
achieved under certain test conditions. Preferably, at room
temperature (27-30.degree. C.), greater than 25 weight % or greater
than 50 weight % of carbon monoxide is oxidized, more preferably
greater than 80 weight % of carbon monoxide is oxidized, even more
preferably greater than 90 weight % of carbon monoxide is oxidized,
and most preferably 100 weight % of carbon monoxide is oxidized
using a gas stream of carbon monoxide in helium or argon, and
oxygen.
It is contemplated that the catalyst may be incorporated into a
cigarette filter component in an amount such that the amount of
carbon monoxide in mainstream smoke is reduced during smoking of a
cigarette. Preferably, the amount of the catalyst will be a
catalytically effective amount, e.g., from about a few milligrams,
for example, about 50 mg/cigarette, to about 350 mg/cigarette. More
preferably, the amount of catalyst will be from about 100
mg/cigarette to about 200 mg/cigarette. Preferably, the catalyst
will be incorporated in a cigarette in an amount effective to
reduce carbon monoxide in mainstream smoke by at least 10%,
preferably at least 20%, 30%, 40%, or 50% or more.
While various embodiments have been described, it is to be
understood that variations and modifications may be resorted to as
will be apparent to those skilled in the art. Such variations and
modifications are to be considered within the purview and scope of
the claims appended hereto.
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